Failure of SVs.pdf

INSPECTION OF PRESSURE-RELIEVING DEVICES 15
CAUTION: When rupture disks are removed for inspection or
when an accompanying relief valve is serviced, the disks can
easily be damaged and can fail prematurely if reused.
Replacement of disks at every maintenance interval will min-
imize the chance of damage and premature failure.
5 Causes of Improper Performance
5.1 CORROSION
Nearly all types of corrosion are present in refinery and
chemical plant services. Corrosion is a basic cause of many of
the difficulties encountered with pressure relief devices. Corro-
sion often appears as: pitted or broken valve parts, deposits of
corrosive residue that interfere with the operation of the mov-
ing parts, or a general deterioration of the material of the reliev-
ing device. Figures 19 through 25 illustrate the effects of
corrosion on relief devices. In addition to internal parts,
exposed studs are vulnerable to environmental corrosion attack.
Corrosion can usually be slowed or stopped by the selec-
tion of more suitable devices or device materials. Proper
maintenance is also a consideration since a leaking valve
allows fluids to circulate in the upper parts of the valve, which
can contribute to the corrosion of its movable parts. Protective
coatings as shown in Figure 26 may offer protection against
corrosion in some services.
In certain applications, a rupture disk device installed on
the inlet or outlet of a pressure relief valve can provide added
corrosion protection of the valve internals.
In many instances, valves of different construction can
avoid, reduce, or even completely contain the effects of corro-
sion. The use of an O-ring seat in a pressure relief valve will
sometimes stop leakage past the seating surface and eliminate
corrosion in the valve's working parts (see Figure 3). However,
O-ring elastomers may have a limited life under stress due to
degradation caused by temperature, aging, or swelling. A bel-
lows seal can be used to protect the spring bonnet cavity and
the discharge side of the valve from the corrosive lading fluid.
(see Figure 5).
5.2 DAMAGED SEATING SURFACES
Because differential loading must be small to prevent leak-
age of the lading fluid, an optical precision on the order of 3
light beads/bands [0.0000348 in. (0.0008838 mm)] must be
maintained in the flatness of seating surfaces on metal-seated
pressure relief valves (see API Std 527). Any imperfection in
these seating surfaces will contribute to improper valve action
in service.
There are many causes of damaged valve seats in refinery
or chemical plant service, including the following:
a. Corrosion.
b. Foreign particles that get into the valve inlet and pass
through the valve when it opens, such as mill scale, welding
spatter or slag, corrosive deposits, coke, or dirt. The particles
may damage the seat contact required for tightness in most
pressure relief valves. The damage can occur either in the shop
during maintenance of the valve or while the valve is in service.
c. Improper or lengthy piping to the valve inlet or obstructions
in the line. These can cause a valve to chatter. The pressure
under the seat may become great enough to open the valve.
However, as soon as the flow is established, the built-up pres-
sure drop in the connecting piping may be so great that the
pressure under the seat falls and allows the valve to close. A
cycle of opening and closing may develop, become rapid, and
subject the valve seating surfaces to severe hammering, which
damages the seating surfaces, sometimes beyond repair. Fig-
ures 27 and 28 show seating surfaces damaged by chattering
and frequent fluctuations of pressure.
d. Careless handling during maintenance, such as bumping,
dropping, jarring, or scratching of the valve parts.
e. Leakage past the seating surfaces of a valve after it has
been installed. This leakage contributes to seat damage by
causing erosion (wire drawing) or corrosion of the seating
surface and thus aggravating itself. It may be due to improper
maintenance or installation such as misalignment of the parts,
piping strains resulting from improper support, or complete
lack of support of discharge piping. Other common causes of
this leakage are improper alignment of the spindle, improper
fitting of the springs to the spring washers, and improper
bearing between the spring washers and their respective bear-
ing contacts or between the spindle and disk or disk holder.
Spindles should be checked visually for straightness. Springs
and spring washers should be kept together as a spring assem-
bly during the life of the spring. Seat leakage may also result
from the operating pressure being too close to the set pressure
of the valve.
f. Improper blowdown ring settings. These can cause chat-
tering in pressure relief valves. The relief valve manufacturer
should be contacted for specific blowdown ring settings for
liquid service and for vapor service.
g. Severe oversizing of the pressure relief valve for the relief
loads encountered can cause the valve to close abruptly,
resulting in disc and nozzle seating surface damage.
5.3 FAILED SPRINGS
Spring failure occurs in two forms. The first is a weakening
of the spring, which causes a reduction in set pressure and the
possibility of premature opening. The second is a total failure
(complete break) of the spring, which causes uncontrolled
valve opening.
Although springs may weaken and fail due to the use of
improper materials in high temperature service, failed springs
are almost always caused by corrosion. Surface corrosion and
stress corrosion cracking are the most prevalent of this type of
failure in refineries.
Copyright American Petroleum Institute
Provided by IHS under license with API Licensee=Shell Global Solutions International B.V./5924979112
Not for Resale, 09/05/2006 02:10:10 MDT
No reproduction or networking permitted without license from IHS

16 API RECOMMENDED PRACTICE 576
Figure 15—Reverse-Acting Scored Rupture Disk
Rupture disk
Pre-assembly side clips
or pre-assembly screws
Standard studs
and nuts
Insert-type
rupture disk holder
(inlet and outlet shown)
Pressure
Inlet
Standard flange
Standard flange
Outlet

Surface corrosion attacks the spring surface until the
cross-sectional area is not sufficient to provide the necessary
closing force. It may also produce pits that act as stress risers
and cause cracks in the spring surface and subsequent spring
failure (see Figure 29).
Stress corrosion cracking sometimes causes rapid spring
failure. It is especially insidious because it is very difficult to
detect before the spring breaks. A brittle-type spring failure
due to stress corrosion cracking is shown in Figure 30.
Hydrogen sulfide (H2S) frequently causes stress-corrosion
cracking of springs (see NACE Standard MR0175 for mate-
rial recommendations and guidance). Consult the manufac-
turer to select an appropriate spring in susceptible
applications since the material strength, hardness and heat
treatment of the spring can affect its resistance to stress corro-
sion cracking.
Where corrosion prevails, three courses of preventive
action may be taken:
a. A spring material that will satisfactorily resist the action of
the corrosive agent may be used.
b. The spring may be isolated by a bellows. Certain pilot-
operated pressure relief valves have diaphragms or pistons
that isolate the pilot spring from the process.
c. The spring may be specially coated with a corrosion-resis-
tant coating that can withstand the operating temperature and
environment.
5.4 IMPROPER SETTING AND ADJUSTMENT
Manuals provided by the valve manufacturer help eliminate
improper setting and adjustment by indicating how to adjust
their valves for temperature, back pressure, and other factors.
Figure 16—Reverse-Acting Rupture Disk with Knife Blades
Rupture disk
Pre-assembly side clips
or pre-assembly screws
Knife-blade or
knife-ring assembly
Standard studs
and nuts
Insert-type
rupture disk holder
(inlet and outlet shown)
Pressure
Inlet
Standard flange
Standard flange
Outlet

In refinery and chemical plant services, setting a pressure
relief valve while it is in place on the equipment to be pro-
tected may be impractical and should be performed only after
special consideration as noted in 6.2.2.18. Generally, direct
spring-loaded valves should be set in the valve maintenance
shop while on appropriate test equipment. During inspection
and repair, a properly designed test block facilitates the setting
and adjusting of the pressure relief valve (see 6.2).
Water, air, or an inert gas such as bottled nitrogen is gener-
ally used as the testing medium in the shop. It is better to set a
pressure relief valve on air, or some other gas, rather than on
water since, depending on the type of valve being tested, a
gaseous effluent will produce either a definite pop or a clearly
defined audible opening at the set pressure. To ensure that the
valve is opening, some overpressure should be carefully
applied because an audible leak could otherwise be misinter-
preted as the result of reaching the set pressure. However,
most pressure relief valves produce a distinct pop at the set
pressure, making misinterpretation unlikely. The size of the
test stand is important since insufficient surge volume might
not cause a distinct pop, and may cause an incorrect set pres-
sure. Vapor service valves should be set using air or inert gas.
Steam service valves should be set using steam, but air may
be used if suitable corrections are applied. Liquid service
valves should be set using water.
Consult the manufacturer for the proper technique for set-
ting pilot-operated pressure relief valves on liquid as the
water in the dome area and pilot assembly may create prob-
lems when placed in service.
Incorrect calibration of pressure gauges is another frequent
cause of improper valve setting. To ensure accuracy, gauges
should be calibrated frequently on a regularly calibrated dead
weight tester. The pressure range of the gauge should be cho-
sen so that the required set pressure of the pressure relief
valve falls within the middle third of the gauge pressure
range. Snubbers on pressure gauges are not generally recom-
mended since they tend to clog and produce pressure lag.
Adjustment of the ring or rings controlling the valve is fre-
quently misunderstood.The valve adjusting ring or rings will
control either the valve blowdown—the difference between
Figure 17—Graphite Rupture Disk
Graphite rupture disk
Standard studs
and nuts
Optional vacuum
support
Pressure
Inlet
Standard flange
Standard flange
Outlet

Figure 18—Combination of Reverse-Buckling Disk
Assembly and Safety Relief Valve
Figure 19—Acid Corrosion in Carbon-Steel Bonnet
Caused by Leaking Seating Surfaces
Excess flow valve
(optional)
Bleed valve
(May be car-sealed open)
Figure 20—Acid Corrosion on 18Cr-8Ni Steel
Inlet Nozzle
Figure 21—Chloride Corrosion on 18Cr-8Ni Steel
Nozzle (with Machined Seating Surface)

the set pressure and the reseating pressure—or valve blow-
down and simmer, depending on the design of the valve being
tested. Because the density and expansion characteristics of
material handled through pressure relief valves are variable
and the volume of testing facilities is limited, it is usually
impractical to adjust the valve rings on a maintenance shop
test block. The rings should therefore be adjusted to obtain a
pop on the valve test drum and then inspected and readjusted
for proper blowdown according to the manufacturer's recom-
mendation. This should permit the best average performance
characteristics of the valve when installed. For liquid or vapor
service, the relief valve manufacturer should be contacted
regarding the proper blowdown ring settings. Full understand-
ing of terminology is important (see ASME PTC 25).
5.5 PLUGGING AND STICKING
In refinery and petrochemical services, process solids such
as coke or solidified products can sometimes plug various
parts of the valve and connected piping. Additionally mono-
mer service can lead to polymer formation and plugging.
Extreme cases of fouling are illustrated by Figure 31, which
shows the inlet nozzle of a valve plugged solid by a mixture
of coke and catalyst, and Figure 32, which shows the outlet
nozzle of a valve plugged with deposits from other valves that
discharge into a common discharge header. Fouling of a
lesser degree, as shown in Figures 33 and 34, is also likely to
impede valve operation. All valve parts, particularly guiding
surfaces, should be checked thoroughly for any type of foul-
ing. Lubricate all load bearing surfaces such as spindle to disk
holder, spring buttons to spindle, disk to disk holder and
threads with a lubricant that is compatible with the process
materials and service temperatures.
Figure 22—Sulfide Corrosion on Carbon-Steel Disk
From Crude-Oil Distillation Unit
Figure 23—Chloride Attack on 18Cr-8Ni Steel Disk Figure 24—Pit-Type Corrosion on 18Cr-8Ni Steel
(Type 316) Bellows

Valve malfunction may also be due to sticking of the disk
or disk holder in the guide which may be caused by corrosion
or galling of the metal or by foreign particles in theguiding
surfaces. Foreign particles in the guiding surfaces tend to roll
metal up, causing severe galling. The use of a bellows can
keep the foreign particles away from the guiding surfaces.
Sticking of valves can also result from machining of valve
parts outside the manufacturer's tolerance limits. Figure 35
shows a disk that is frozen in the guide as a result of corrosion
in sour gas service. If corrosion is the cause of the sticking,
three possible cures are available. First, the use of a bellows
can protect moving parts from the corrosive substance, espe-
cially in closed systems (see Figure 4). Second, an O-ring
seat (see Figure 3) can seal the guiding surfaces from the lad-
ing ﬂuid until a relief cycle occurs. Third, the use of a rupture
Figure 25—Monel Rupture Disks Corroded in Sour-Gas Service
Figure 26—Body and Bonnet Coated With Epoxy for
Corrosion Protection
Figure 27—Seating Surface of Disk Deformed by
Chattering
Figure 28—Seat and Disk Damaged by Frequent
Operation of Valve Set Too Close to Operating Pressure

disk on the valve inlet will isolate the valve internals from the
upstream process material.
When galling of the metal in the guiding surfaces is not due
to corrosion or foreign particles, it is often due to valve chatter
or flutter caused by improper piping at the valve inlet or outlet
or by severe oversizing of the valve. Correction of improper
piping at the valve inlet or outlet will usually stop galling.
Improper finishing of the guiding surfaces can also cause gall-
ing caused by chatter or flutter. To reduce the chances of gall-
ing, they should be polished until they are as smooth as
possible. Varying the materials and hardness of the contacting
parts until the best combinations are found may minimize
galling. Consult valve manufacturer for recommendations.
Sticking of pressure relief valves may also be caused by
poor alignment of the valve disk, which is usually due to
debris on the contact surface between the guide and the body
of the valve, or misalignment of a gasket at assembly (see
Part II of API RP 520).
5.6 MISAPPLICATION OF MATERIALS
In general, the temperature, pressure, corrosion resistance
requirements and the atmospheric conditions of the service
determine the materials required for a pressure-relieving
device in a given service. The selection of standard valves
that meet those requirements and are appropriate for those
conditions is advisable. Occasionally, however, severe corro-
sion or unusual pressure or temperature conditions in the pro-
cess require special consideration. Manufacturers can usually
supply valve designs and materials that suit special services.
Catalogs show a wide choice of special materials and acces-
sory options for various chemical and temperature conditions.
Addition of a rupture disk device at the inlet or outlet may
help prevent corrosion.
The hydrogen sulfide (H2S) attack on the carbon steel
spring in Figure 22 and the chloride attack on the 18Cr-8Ni
steel disk in Figure 23 exemplify the results of the misappli-
cation of materials. Where service experience indicates that a
selected valve type or material is not suitable for a given ser-
vice condition, an immediate correction that will ensure
dependable operation should be made. Great care should be
taken to record the identity of special materials and the loca-
tions requiring them. An adequate system of records should
provide the information needed for the repair or recondition-
ing of valves in special service and for developing optimum
purchase specifications.
Figure 29—Spring Failure Due to Corrosion
Figure 30—Spring Failure Due to Stress Corrosion
Figure 31—Inlet Nozzle Plugged with Coke and
Catalyst After Nine Months in Reactor Vapor Line

5.7 IMPROPER LOCATION, HISTORY, OR
IDENTIFICATION
If not installed at the exact location for which it was
intended a pressure relief valve might not provide the proper
protection. To assist in the identification of valves and to pro-
vide information necessary for correct repairs and installa-
tion, a comprehensive set of specification and historical
records should be maintained and referred to when valves are
removed for inspection and repair. Most pressure relief valves
have an identifying serial or shop number placed on the valve
by the manufacturer or an identifying number tagged,
stamped, or otherwise placed on the valve by the user. Some
users also stamp mating pipe flanges with device numbers.
This identification specifies the location of the valve and, by
reference to the specification record, its limitations and con-
struction (see Section 7).
5.8 ROUGH HANDLING
Because of the difficulty in obtaining absolute tightness in
most pressure-relieving devices, valves are manufactured
according to a commercial tightness standard (see API Std
527). Valves are checked for tightness in the manufacturer's
plant before they are shipped to the user. Valve tightness is
sometimes checked by the user in the maintenance shop
before initial use and usually checked after subsequent clean-
ing, repairing, or testing. Subsequent rough handling of the
valve, however, can change the set pressure, damage lifting
levers, damage tubing and tubing fittings, damage pilot
assemblies or cause internal or external leakage when the
valve is in service. Rough handling can occur during ship-
ment, maintenance, or installation.
5.8.1 During Shipment
Because of their operation, most pressure relief valves have
a sturdy appearance that may obscure the fact that they are
delicate instruments with very close tolerances and sensitive
dimensions. Accordingly, commercial carriers sometimes
subject them to rough handling. This may cause a valve to
leak excessively in service or during testing. This rough han-
dling may also expose the valve inlet to dirt or other foreign
particles that could damage the valve seating surface the first
time the valve opens and cause leakage thereafter.
Pressure relief valves should be shipped in an upright posi-
tion—this is especially true of large valves and valves with
low set pressures. When large, low-pressure valves are
allowed to lie on their sides, the springs may not exert the
same force all around the seating surfaces.
Figure 32—Outlet Valve Plugged with Deposits From
Other Valves in Common Discharge Header
Figure 33—Moving Parts of Valve Fouled with Iron
Sulfide (FeS2)

5.8.2 During Maintenance
Pressure relief valve parts are usually precision items man-
ufactured to extremely close tolerances. Rough handling can
degrade these tolerances. Rough handling can also destroy
the basic valve alignment on which the ﬁne, exacting perfor-
mance characteristics of the device primarily depend. Careful
handling of the valve during all phases of maintenance is
important. Both before and after repairs, rough handling of
the completely assembled valve should be avoided. Before
the valves leave the shop, valve inlets and outlets should be
covered.
Rough handling during maintenance includes application
of excessive back pressure, which should not be applied to a
bellows valve during a maintenance-related test.
5.8.3 During Installation
Valve inlets and outlets should have been covered before
the valves left the shop. If they were not covered when
received for installation, provisions should be made to ensure
that in the future they are covered before leaving the shop.
Pressure relief valves should be transported in an upright
position.
Rough handling of a pressure relief valve by personnel
during installation may cause poor valve performance in ser-
vice. Bumping or dropping the valve should be carefully
avoided. The valves shown in Figure 36 were dropped from
the bed of a truck after being repaired. As a result, they leaked
once they were installed.
5.9 IMPROPER DIFFERENTIAL BETWEEN
OPERATING AND SET PRESSURES
The differential between operating and set pressures pro-
vides seat loading to keep the pressure relief valve tightly
closed. Due to a variety of service conditions and valve
designs, only general guidelines can be given for designing a
system. NB-23 and Sections VI andVII of theASME Code are
useful references. However, individual applications and experi-
ence must ultimately be relied on. Although greater differen-
tials between operating and set pressures promote trouble-free
Figure 34—Valve Stuck Because of Iron Sulﬁde
(FeS2) Deposits
Figure 35—Disk Frozen in Guide Because of Buildup
of Products of Corrosion in Sour-Oil Vapor Service
Figure 36—Rough Handling of Valves Should
Be Avoided

operation, they may also increase the cost of the equipment.
Inspections should record operating experience and provide
feedback to be considered in future design and remedial action.
5.10 IMPROPER DISCHARGE PIPING TEST
PROCEDURES
When hydrostatic tests of discharge piping are performed,
blinds must be installed. Otherwise, results such as the fol-
lowing might occur:
a. The disk, spring, and body area on the discharge side of
the valve are fouled.
b. The bellows of a balanced relief valve are damaged by
excessive back pressure.
c. The dome area and/or pilot assembly of a pilot-operated
pressure relief valve are fouled and damaged by the backflow
of fluid.
6 Inspection and Testing
6.1 REASONS FOR INSPECTION
Pressure-relieving devices are installed on process equip-
ment to release excess pressure due to operational upsets,
external fires, and other hazards. These hazards are discussed
in API RP 520 and API RP 521. Failure of pressure-relieving
devices to function properly when needed could result in the
overpressure of the vessels, exchangers, boilers, or other
equipment they were installed to protect. A properly
designed, applied, and installed pressure-relieving device that
is maintained in good operating condition is essential to the
safety of personnel and the protection of equipment during
abnormal circumstances. One of the principal reasons for
inspecting pressure-relieving devices is to ensure that they
will provide this protection.
Inspections of pressure relief devices must determine the
general physical and operating conditions of the devices, and
ensure that their performance meets the requirements for a
given installation. In making this determination, two types of
inspections can be used. They are "shop inspection/over-
hauls", and "visual on-stream inspections". Pre- and post-test-
ing of the pressure-relieving device should be included in the
“shop inspection/overhaul”. Each is discussed in the follow-
ing sections.
6.2 SHOP INSPECTION/OVERHAUL
Periodically, pressure relief devices will be removed, disas-
sembled, and inspected. These inspections are referred to as
"shop inspection/overhaul" (although some, if not all of the
work can be performed in the field). Also, while the device is
removed, inlet and outlet piping should be inspected for the
presence of internal deposits, and records should be kept of
their condition. If heavy fouling is observed, the piping
should be cleaned. If necessary, piping should be dismantled
for inspection and cleaning.
6.2.1 Safety
Before inspection and any repairs on pressure-relieving
devices are executed, general precautions should be taken to
maintain the safety of the equipment protected by the devices,
especially if the equipment is in operation. When inspection
and repairs on an operating unit are required, the unit opera-
tions should be normal and the proper authority and permits
for the work should be obtained.
Some pressure-relieving valves have set pressures that
exceed their outlet flange rating. If these valves are equipped
with outlet block valves, the pressure relief valve inlet block
valve should be closed before the outlet valve is closed. Also,
the pressure relief valve body must be vented immediately
after the outlet isolation block valve is closed. This prevents
high pressures from the pressure relief valve inlet from possi-
bly overpressuring the pressure relief valve body. Similar
caution must be exercised when installing a blind in the pres-
sure relief valve outlet. Installation of drain valves between
the inlet and outlet block valves and the pressure relief valve
may be considered.
Before disconnecting pressure-relieving devices, the con-
nected piping and block valves should be checked to ensure
that they are sufficiently supported. After reinstalling pressure
relief valves, the related piping should be checked to ensure
that it is not imposing loads that would cause problems with
the pressure relief valve body such as distortion leading to
in-service leakage.
Some devices may trap hazardous toxic process material in
bonnet cavities or dome cavities. Special steps during decon-
tamination should be taken to minimize exposure of shop per-
sonnel.
6.2.2 Valve Identification
To minimize errors in the testing and handling of pressure
relief valves, each should carry an identifying tag, stencil,
plate, or other means to show its company equipment num-
ber. This number allows ready identification of the device's
unit, the equipment that the device should be installed on, the
device's set pressure, and the date of its last test (see Figure 37
for an example of an identifying tag). If a relief device cannot
already be easily and correctly identified by a marking on it, it
should be marked and identified as described above before it
is removed from its equipment. Also see ASME Section VIII,
Division 1, paragraph UG-129 for instructions on marking
nameplates of pressure relieving devices.

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